Adding multiple green screenplates into an animated 3D environment
Your best bet would be to add the green screen video as a video texture on a plane object. You would need to create a black and white alpha mask clip as well to drive the transparency on the material node. This way you can have a plane object for each person in the scene and place them in the scene as needed.
Here's a test scene I just put together using this Minion green screen footage here: https://www.youtube.com/watch?v=QjN0BsUqeb4
I used blender's compositor to key out the green screen and create a black and white alpha matte video file (it's not a perfect key, but good enough for this demo):

Then I imported the green screen video use the Import Image as Plane feature. I find setting the image material to Emission works better than Diffuse in this scenario.
Then I set up the material to use the BW video file as the factor input for the Mix node that is mixing a transparent shader and the colour emission shader.

Final Output from Scene:

I should also mention that another advantage of handling the transparency via the Material as opposed to doing it in the Compositor after rendering is that you can see the character properly in the viewport when setting up your scene. This is very helpful in the pre-rendering stage for scene development.